Bacteria Make Female Butterflies Promiscuous, Scientists Say

The common eggfly (Hypolimnas bolina) is often infected with a male-killing germ known as Wolbachia. Scientists now find male deaths can trigger a vicious circle of increasing female promiscuity and resulting male sexual exhaustion.Credit: Sylvain Charlat

A
germ that kills males triggers a vicious cycle of increasing female promiscuity
and male sexual exhaustion in a species of butterfly, scientists report.

Male-killing
bacteria known as Wolbachia are extremely widespread in insects, found in more than
one-fifth of species. The germs can turn males to females and cause infected
females to reproduce
without males.

Scientists
had assumed these bacteria
would profoundly alter the natural mating
patterns of their hosts, but only had scant evidence of what these changes would
entail in the wild.

Evolutionary
biologist Sylvain Charlat at University College London and his colleagues
investigated the common
eggfly Hypolimnas bolina. This butterfly is found in locations
ranging from Madagascar to Asia, and from Australia and to Easter Island.

The
bacteria infects Pacific Island and Southeast Asian populations of the
butterfly [image],
getting transmitted from mother to son and killing males before eggs
hatch. Each island's butterflies
are affected by Wolbachia differently, leading to different ratios of
males to females. The male population can range as low as one male to every 100
females in some areas.

Over
the course of three years, the scientists inspected the butterflies' sex ratio
in 20 different locales, including Vietnam, Australia and 18 different islands,
including Borneo, New Guinea, Vanuatu and Tahiti. They also investigated female
mating frequency and the size of the male sperm
package.

Some
research sites were easily reachable by airplane, but the scientists relied on
private sailboats to get to the more remote spots. While butterflies were
common at some locations, they were rare at others, requiring days and days of
hiking to find spots for collection. "People were generally very curious
about what I was doing, and amused when they knew it was all about sex in
butterflies," Charlat said

The
researchers expected that the fewer male butterflies
there were, the less sex females likely would have with males. Surprisingly,
female promiscuity
actually rose.

"Greater
numbers of female partners leads to fatigue in males. They start producing
smaller sperm packages," Charlat said. "Unfortunately, the female
butterflies instinctively know that the packages are smaller and that their
chances of having been sufficiently impregnated after mating are lower than
usual. This just makes them more rampant."

The
actual mechanism behind how the females detect sperm
package size remains a mystery so far.

The
fact that the Wolbachia bacteria are widespread in insects could mean, Charlat
speculated, that this phenomenon might also be widespread in insects
in nature.

Charlat
and his colleagues reported their findings in the Feb. 6 issue of the journal Current
Biology.

Charles Q. Choi

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.